An important consideration in the electrical and packaging design of integrated circuits is the number of input and output pins provided for communication between the integrated circuit and external circuitry with which a circuit must operate. As a general rule, the cost of a package of an integrated circuit is directly proportional to the number of input/output (I/O) pins. Additionally, packages with larger numbers of pins usually occupy a greater area of circuit board or other mounting medium for the chip. Consequently, it is desirable to utilize circuit pins for more than one function if possible.
With the advent of very large scale integrated (VLSI) circuit devices, costs associated with testing the devices for operability and reliability represent a significant portion of manufacturing costs due to increased complexity of VLSI devices. Designers of integrated circuits can reduce testing costs by providing access to the circuitry beyond normal user access in order to directly test certain critical functions. To allow such access, some designs may include a "test mode" wherein certain features beyond typical user features can be made available to the user. Access to predetermined portions of circuitry may be provided without increasing the total pin count of the package by using a pin (or several pins) for more than one function.
A designated pin may be provided with a detection means, for example, which detects whether or not a voltage applied to the pin is outside the voltage range expected in normal use. An example of a detection circuit for providing this function is taught by James S. Thomas et al. in U.S. Pat. No. 4,318,013 entitled "High Voltage Detection Circuit". The detection of such a voltage can be used to place the circuit in a "test" mode in which the circuit may allow operations different from those allowed in the normal or "user" mode.
The detection circuit in such a device must provide an unambiguous indication of the chip mode upon the application of the appropriate voltage. In the past such circuits have had varying degrees of sensitivity to supply voltages and/or to processing variations.